Currently, the resins of choice for photovoltaic encapsulants, signing overlay films, and similar outdoor applications are polymethyl methacrylate (PMMA), impact-modified PMMA, PMMA blends, and polyethylene terephthalate (PET). While these films have many desirable mechanical properties, including a high impact resistance, they also have high coefficients of hygroscopic expansion (CHE) (see Table 1). Consequently, all of these films are prone to dimensional distortions in high temperature, high humidity environments. Furthermore, many of these films are prone to moisture induced hydrolysis reactions. See, for example, "Concise Encyclopedia of Polymer and Science Engineering", pp. 1307-09 (1990). Thus, while PMMA has a relatively high (about 105.degree. C.) glass transition temperature (T.sub.g) under normal conditions, its Tg is effectively lowered in high humidity environments due to water plasticization. PET has similar difficulties with moisture absorption that leads to suppression of an already lower T.sub.g (68.degree. C.) and molecular weight degradation via hydrolysis.
TABLE 1 ______________________________________ Moisture Absorption and CHE Data for sPS, PMMA, and PET Moisture Absorption Coefficient of Hygroscopic Film (%) Expansion (ppm/% RH) ______________________________________ sPS 0.03 0.5 PMMA 1.5 15 PET 0.5 10 ______________________________________
Many films currently used in outdoor applications are also subject to staining from both natural sources, such as dirt, and artificial sources, such as graffiti. To combat this, several water-based, cross-linkable, fluorochemical low surface energy coating systems have been made from polymeric surfactants and oxazoline polymer crosslinkers. These materials are described, for example, in U.S. Pat. Nos. 5,382,639, 5,294,662, 5,006,624, and 4,764,564.
A further problem encountered with the use of polymeric films in outdoor applications concerns their exposure to UV radiation. Polymeric films which are subjected to long-term exposure to ultraviolet (UV) radiation have a tendency to become brittle and yellowed over time due to the onset of photo-oxidation. This is so even if the polymer itself is transparent in the UV region of the spectrum, since UV radiation may be absorbed by metal catalysts and other impurities remaining from the manufacturing process. See, for example, R. Hirt et al., SPE Trans, Vol. 1, 1 (1961). Consequently, most polymeric films which are to be used in signing and other outdoor applications are stabilized against UV degradation by compounding the base resin with UV absorbing (UVA) additives and/or other compounds that act as excited state quenchers, hydroperoxide decomposers, or free radical scavengers. Hindered-amine light stabilizers (HALS) have been found to be particularly good radical scavengers. UVA additives act by absorbing radiation in the UV region of the spectrum. HALS, on the other hand, behave by quenching radicals generated within the polymer matrix during exposure to UV radiation. A review of the types of materials used to improve UV stability may be found in R. Gachter, H. Muller, and P. Klemchuk (Editors), Plastics Additives Handbook, pp. 194-95 (3rd Ed., published by Hanser Publishers, New York).
In some instances, UV-sensitive substrates have also been protected with a UV-absorbing topcoat. Thus, PCT/US93/05938 discloses aqueous latices of UV-absorbing polymers that can be applied as topcoats on UV-sensitive substrates. Similarly, H. Razavi et al., "Paradigm Shift In UV Protection Technology" (paper presented at the SPE Conference, Oct. 12, 1993) describe a UV absorbing top coat wherein the UV absorber is covalently bonded to the backbone of a carrier polymer.
While work on UV-stabilizers has progressed, much work has also been done on the development of polymers having specific tacticities. Thus, recent developments in catalysis technology have enabled the synthesis of stereoisomers of polystyrene having a predominantly syndiotactic configuration. This material, referred to herein as syndiotactic polystyrene (sPS), has been used to produce various articles which exhibit good dimensional stability and/or heat or moisture resistance. Such references include U.S. Pat. No. 5,496,919 (Nakano), U.S. Pat. No. 5,188,930 (Funaki et al.), U.S. Pat. No. 5,476,899 (Funaki et al.), U.S. Pat. No. 5,389,431 (Yamasaki), U.S. Pat. NO. 5,346,950 (Negi et al.), U.S. Pat. No. 5,318,839 (Arai et al.), U.S. Pat. No. 5,273,830 (Yaguchi et al.), U.S. Pat. No. 5,219,940 (Nakano), U.S. Pat. No. 5,166,238 (Nakano et al.), U.S. Pat. No. 5,145,950 (Funaki et al.), U.S. Pat. No. 5,127,158 (Nakano), and U.S. Pat. No. 5,082,717 (Yaguchi et al.).
The high dimensional stability of sPS films in high temperature/high humidity environments would make them desirable as overlay films for signing and other outdoor applications. However, sPS films are prone to UV-induced molecular weight breakdown and yellowing, phenomena which are often attributed to the presence of UV absorbing catalyst residues, peroxide groups in the polymer chain, and/or oxidation products formed during high temperature processing. Furthermore, as with other polystyrenes, sPS has inherent photochemical instabilities due to the large number of activated tertiary hydrogens. These hydrogens have a propensity to be photochemically extracted, thereby initiating free radical degradation pathways.
Some attempts have been made to improve the resistance of sPS to UV radiation. Thus, U.S. Pat. No. 5,496,919 (Nakano) discloses the preparation of articles based on sPS by compounding the sPS base resin with various additives such as antioxidants and UV absorbers. However, the use of UVAs and HALS in sPS films is found to only moderately improve their long-term outdoor weatherability. To date, the failure to produce sPS films that are stabilized against UV radiation has thwarted the development of commercial overlay films based on sPS for signing and other outdoor applications.
It is thus an object of the present invention to provide an sPS overlay film which has good dimensional stability, is resistant to UV degradation, and is suitable for use in signing and other outdoor applications. This and other objects are achieved by the present invention, as hereinafter described.